JCLOUDS-588: Register discovered images in the image cache

Images were cached in memory using a memoized supplier. To allow growing
this cache with the discovered images, the ImageCacheSupplier class has
been created. It provides an in-memory cache with all discovered images
and acts as a view over the image cache that also provides access to
them.

The in-memory cache for the discovered images expires with the session,
just as the image cache does.

The default memoized image supplier has been changed to the
ImageCacheSupplier, to make sure all providers get injected the right
instance, and the old supplier has been qualified with the 'imageCache'
name, in case a provider needs the basic image cache.